Optical receiver, active optical cable, and control method for optical receiver

ABSTRACT

The present disclosure includes a photodetector element (11) that converts an optical signal into an electric current signal; a transimpedance amplifier (12a) that converts the electric current signal into a voltage signal; a differential amplifier (12d) that converts the voltage signal into a differential signal, by performing differential amplification of a difference between the voltage signal and a threshold voltage; an LOS detection circuit that detects a no-signal section of the optical signal; and an MCU that repeatedly executes offset cancellation processing, the offset cancellation processing including threshold voltage change processing in which the threshold voltage is changed such that an offset voltage of the differential signal is reduced, the MCU 13 skipping the threshold voltage change processing in the no-signal section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/170,185, filed on Jun. 1, 2016, which is a Continuation of PCTInternational Application No. PCT/JP2015/079191 filed in Japan on Oct.15, 2015, which claims the benefit of Patent Application No. 2014-211225filed in Japan on Oct. 15, 2014, the entire contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to (i) an optical receiver that receivesan optical signal, converts the optical signal thus received into avoltage signal, and then outputs the voltage signal outside the opticalreceiver, (ii) an active optical cable including such an opticalreceiver, and (iii) a control method for such an optical receiver.

BACKGROUND ART

Active optical cables have been garnering attention as a transmissionmedium to serve as an alternative to metal cables. An active opticalcable is configured such that it includes (i) a cable containing anoptical fiber; and (ii) a pair of connectors respectively provided onboth ends of the cable. One of the connectors functions as an opticaltransmitter. This optical transmitter converts, into an optical signal,a voltage signal externally supplied (e.g., from a data center computer)and then transmits the optical signal. The other connector serves as anoptical receiver. This optical receiver converts a received opticalsignal into a voltage signal and then outputs the voltage signal outsidethe optical receiver (e.g., to a data center storage device). By makingeach of the connectors function as both an optical receiver and anoptical transmitter, it is possible to realize bidirectionalcommunication using the active optical cable.

FIG. 9 illustrates an optical receiver 2. The optical receiver 2 is aconventional optical receiver that can be used as a connector for anactive optical cable. The optical receiver 2 includes (i) aphotodetector element 21 that converts an optical signal into anelectric current signal, and (ii) a receiving circuit 22 that convertsthe electric current signal into a voltage signal.

The receiving circuit 22 is configured such that it includes atransimpedance amplifier 22 a, differential amplifiers 22 b through 22e, a low-pass filter 22 f, and an error amplifier 22 g.

The transimpedance amplifier 22 a converts, into a voltage signal(single end), an electric current signal outputted from thephotodetector element 21. The differential amplifier 22 b performsdifferential amplification of the difference between (i) a voltagesignal outputted from the transimpedance amplifier 22 a and (ii) athreshold voltage Vth. By performing this differential amplification,the differential amplifier 22 b obtains a differential signal consistingof a positive phase signal and a negative phase signal. The group ofdifferential amplifiers 22 c through 22 e performs differentialamplification of the differential signal outputted from the differentialamplifier 22 b.

If the output voltage of the transimpedance amplifier 22 a is Vtia, thenthe differential amplifier 22 b has a positive phase output voltage V1 pwhich is expressed as V1 ocm+a1×(Vtia−Vth)/2, and a negative phaseoutput voltage Vln which is expressed as V1 ocm−a1×(Vtia−Vth)/2. Here,V1 ocm is an output common mode voltage (a predetermined value) of thedifferential amplifier 22 b, and a1 is a gain (a predetermined value) ofthe differential amplifier 22 b.

An average value of a high level and a low level of a voltage signaloutputted by the transimpedance amplifier 22 a is, hereinafter, alsoreferred to as an “average output level of transimpedance amplifier 22a.” In a case where the average output level of the transimpedanceamplifier 22 a is equal to a threshold voltage Vth, the positive phasesignal and the negative phase signal outputted from the differentialamplifier 22 b have waveforms which become symmetrical to each otherwith respect to the output common mode voltage V1 ocm. However, in acase where the power of the optical signal being received fluctuates andthe transimpedance amplifier 22 a has an average output level that isnot equal to the threshold voltage Vth, the positive phase signal andthe negative phase signal have respective waveforms which becomeasymmetrical to each other with respect to the output common modevoltage V1 ocm. This sort of asymmetry causes distortion of a waveformof an output signal of the optical receiver 2.

The low-pass filter 22 f and the error amplifier 22 g are each acomponent for avoiding the above problem. The low-pass filter 22 fperforms smoothing of (i) the positive phase signal outputted from thedifferential amplifier 22 c and (ii) the negative phase signal outputtedfrom the differential amplifier 22 c. The error amplifier 22 g receives:(i) a smoothed positive phase signal (DC component of the positive phasesignal) outputted from the low-pass filter 22 f, and (ii) a smoothednegative phase signal (DC component of the negative phase signal)outputted from the low-pass filter 22 f. The error amplifier 22 g thenintegrates the difference between the respective values of these twosmoothed signals, i.e., an offset voltage of the differential signaloutputted from the differential amplifier 22 c. A resulting integrationvalue of the offset voltage outputted from the error amplifier 22 g isfed back, as a threshold voltage Vth, into a negative phase input of thedifferential amplifier 22 b.

The integration value of the offset voltage outputted from the erroramplifier 22 g follows the average output level of the transimpedanceamplifier 22 a. As a result, even if the power of the optical signalbeing received fluctuates, the abovementioned distortion problem isavoided.

Patent Literature 1 is an example literature disclosing art that cancelsan offset voltage of a differential signal.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication, Tokukai, No. 2008-109559(Publication Date: May 8, 2008)

SUMMARY OF INVENTION Technical Problem

Though the aforementioned distortion problem is avoided as describedabove, conventional optical receivers such as the optical receiver 2illustrated in FIG. 9 exhibit the problem that immediately aftertransition of an optical signal from a no-signal section to a signalsection, a waveform of an output signal is distorted. The followingdiscussion provides a more detailed explanation of this problem, withreference to FIG. 10.

(a) of FIG. 10 is a waveform chart of an optical signal which isreceived by the photodetector element 21. (b) of FIG. 10 is a waveformchart of a voltage signal which is outputted by the transimpedanceamplifier 22 a. Note that (b) of FIG. 10 also shows a dotted linerepresenting a change, over time, of the threshold voltage Vth outputtedby the error amplifier 22 g.

The optical signal shown in (a) of FIG. 10 has a pattern consisting of(i) a DATA section (signal section) whose value alternates between ahigh level and a low level, and (ii) an IDLE section (no-signal section)whose value is constantly an off level. When the photodetector element21 receives the optical signal shown in (a) of FIG. 10, thetransimpedance amplifier 22 a outputs the voltage signal shown in (b) ofFIG. 10.

As shown in (b) of FIG. 10, an average output level of thetransimpedance amplifier 22 a differs between the DATA section and theIDLE section. Due to this difference, distortion occurs in the waveformof the output signal of the optical receiver 2. This distortion occursfrom the point in time at which the IDLE section has transitioned to theDATA section until the point in time at which the output voltage Vth ofthe error amplifier 22 g catches up to the average output level of thetransimpedance amplifier 22 a in the DATA section.

Note that in a link-up sequence of serial communication in conformancewith SAS (Serial Attached SCSI), an OOB (Out of Band) signal may be sentor received, the OOB signal having a pattern consisting of (i) a DATAsection whose value alternates between a high level and a low level, and(ii) an IDLE section whose value is constantly an intermediate levelbetween the high level and the low level. Similarly, in serialcommunication in conformance with PCIe (PCI Express), a signal may besent or received, the signal having a pattern consisting of (i) a DATAsection in which a high level and a low level repeatedly alternate, and(ii) an EI (Electrical Idle) section in which an intermediate level ismaintained. Consequently, using an active optical cable to realizeserial communications in conformance with the above two standardsrequires sending and receiving an optical signal having a pattern suchas the one shown in (a) of FIG. 10, for example.

The present invention has been accomplished in view of the aboveproblem. An objective of the present invention is to realize an opticalreceiver capable of performing offset cancellation while causing nodistortion of a waveform of an output signal immediately aftertransition from a no-signal section to a signal section.

Solution to Problem

In order to solve the above problem, an optical receiver in accordancewith the present invention includes: a photodetector element thatconverts an optical signal into an electric current signal; atransimpedance amplifier that converts the electric current signal intoa voltage signal; a differential amplifier that converts the voltagesignal into a differential signal, by performing differentialamplification of a difference between the voltage signal and a thresholdvoltage; a no-signal detection circuit that detects a no-signal sectionof the optical signal; and a control section that repeatedly executesoffset cancellation processing, the offset cancellation processingincluding threshold voltage change processing in which the thresholdvoltage is changed such that an offset voltage of the differentialsignal is reduced, the control section skipping the threshold voltagechange processing in the no-signal section.

As another way to solve the above problem, a control method for anoptical receiver according to the present invention is A control methodfor an optical receiver, the optical receiver including: a photodetectorelement that converts an optical signal into an electric current signal;a transimpedance amplifier that converts the electric current signalinto a voltage signal; and a differential amplifier that converts thevoltage signal into a differential signal, by performing differentialamplification of a difference between the voltage signal and a thresholdvoltage, the control method comprising the steps of: (i) detecting ano-signal section of the optical signal; and (ii) performing control byrepeatedly executing offset cancellation processing, the offsetcancellation processing including threshold voltage change processing inwhich the threshold voltage is changed such that an offset voltage ofthe differential signal is reduced, the step (ii) skipping the thresholdvoltage change processing in the no-signal section of the opticalsignal.

Advantageous Effects of Invention

With the present invention, it is possible to perform offsetcancellation while causing no distortion of a waveform of an outputsignal immediately after transition from a no-signal section to a signalsection.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an opticalreceiver according to one embodiment of the present invention.

FIG. 2 is a flowchart showing a flow of offset cancellation processingexecuted by an MCU of the optical receiver illustrated in FIG. 1.

FIG. 3 is a flowchart showing a flow of processing, in its entirety,executed by an MCU of the optical receiver illustrated in FIG. 1.

FIG. 4 shows one example of an operation of the MCU, which operation isrealized by determining the timing of execution of the offsetcancellation processing in accordance with the flowchart shown in FIG.3.

FIG. 5 is a graph showing the relationship between (i) received power(dBm) and (ii) the amount of change in variable Vth (in steps) necessaryto cancel offset for the received power. This graph shows cases where aninitial value of the variable Vth is set to a maximum value and aminimum value, respectively.

FIG. 6 is a flowchart showing a flow of threshold voltage changeprocessing executed by an MCU of the optical receiver illustrated inFIG. 1.

FIG. 7 shows one example of an operation of an MCU, which operation isrealized by executing threshold voltage change processing in accordancewith the flowchart shown in FIG. 6.

FIG. 8 is a block diagram illustrating an active optical cable that isprovided with the optical receiver illustrated in FIG. 1.

FIG. 9. is a block diagram illustrating a configuration of aconventional optical receiver.

(a) of FIG. 10 is a waveform chart of an optical signal received by aphotodetector element of the optical receiver illustrated in FIG. 9. (b)of FIG. 10 is a waveform chart of a voltage signal outputted by atransimpedance amplifier of the optical receiver illustrated in FIG. 9.

DESCRIPTION OF EMBODIMENTS

[Configuration of Optical Receiver]

The following discusses a configuration of an optical receiver 1 withreference to FIG. 1. FIG. 1 is a block diagram illustrating aconfiguration of the optical receiver 1.

The optical receiver 1 is a device that receives an optical signal,converts the optical signal thus received into a voltage signal (in thepresent embodiment, a differential voltage signal), and outputs, outsidethe optical receiver 1, the signal thus converted. As illustrated inFIG. 1, the optical receiver 1 includes a photodetector element 11, areceiving circuit 12, an MCU (Micro Controller Unit) 13, a nonvolatilememory 14, and a reference voltage source 15.

The photodetector element 11 is a component for receiving an opticalsignal and converting the optical signal thus received into an electriccurrent signal. In the present embodiment, the photodetector element 11is a photodiode (PD). The electric current signal obtained by thephotodetector element 11 is supplied to the receiving circuit 12.

The receiving circuit 12 is a component for converting the electriccurrent signal obtained by the photodetector element 11 into adifferential voltage signal (hereinafter, referred to as “differentialsignal”). As illustrated in FIG. 1, the receiving circuit 12 includes atransimpedance amplifier 12 a, a variable voltage source 12 b, a dummytransimpedance amplifier 12 c, differential amplifiers 12 d through 12g, a low-pass filter 12 h, a switch 12 i, an error amplifier 12 j, aLoss of Signal (LOS) detection circuit 12 k, and an I2C interface 12 m.

The transimpedance amplifier 12 a converts the electric current signalobtained by the photodetector element 11 into a voltage signal (singleend). An output voltage Vtia of the transimpedance amplifier 12 a issupplied to a negative phase input terminal of the differentialamplifier 12 d.

The variable voltage source 12 b generates a threshold voltage Vth. Alevel of the threshold voltage Vth thus generated can be controlled bythe MCU 13 through the I2C interface 12 m. The threshold voltage Vthgenerated by the variable voltage source 12 b is supplied to a positivephase input terminal of the differential amplifier 12 d.

Note that in the present embodiment, the dummy transimpedance amplifier12 c is interposed between the variable voltage source 12 b and aground. This interposition is intended to cancel (i) temperaturedependence of the transimpedance amplifier 12 a and (ii) power supplyvoltage dependence of the transimpedance amplifier 12 a (i.e., acomponent of voltage that is included in the output voltage of thetransimpedance amplifier 12 a and is not dependent on photoelectriccurrent Ipd that flows through the photodetector element 11).

The differential amplifier 12 d converts the voltage signal outputtedfrom the transimpedance amplifier 12 a into a differential signal. Thedifferential amplifier 12 d achieves this conversion by performingdifferential amplification of the difference between (i) the voltagesignal outputted from the transimpedance amplifier 12 a, and (ii) thethreshold voltage Vth. A positive phase output voltage V1 p of thedifferential amplifier 12 d is expressed as V1 ocm+a1×(Vtia−Vth)/2. Anegative phase output voltage V1 n of the differential amplifier 12 d isexpressed as V1 ocm−a1×(Vtia−Vth)/2. Here, V1 ocm is an output commonmode voltage (a predetermined value) of the differential amplifier 12 d,and a1 is a gain (a predetermined value) of the differential amplifier12 d. The differential signal obtained by the differential amplifier 12d is supplied to a group of differential amplifiers 12 e through 12 g.

The group of differential amplifiers 12 e through 12 g amplifies thedifferential signal obtained by the differential amplifier 12 d. Adifferential signal (a positive phase output voltage V2 p and a negativephase output voltage V2 n) outputted from the differential amplifier 12e, i.e., a first-stage differential amplifier in the group, is suppliedto the low-pass filter 12 h. A differential signal (a positive phaseoutput voltage V4 p and a negative phase output voltage V4 n) outputtedfrom the differential amplifier 12 g, i.e., from a last-stagedifferential amplifier in the group, is outputted outside the opticalreceiver 1.

The low-pass filter 12 h performs smoothing of (i) the positive phasesignal outputted from the differential amplifier 12 e to give a smoothedpositive phase signal and (ii) the negative phase signal outputted fromthe differential amplifier 12 e to give a smoothed negative phasesignal. The smoothed positive phase signal, i.e., a DC component <V2 p>of the positive phase signal outputted from the differential amplifier12 e, is supplied to a positive phase input terminal of the erroramplifier 12 j. On the other hand, the smoothed negative phase signal,i.e., a DC component <V2 n> of the negative phase signal outputted fromthe differential amplifier 12 e, is supplied to a negative phase signalinput terminal of the error amplifier 12 j.

The error amplifier 12 j performs differential amplification of thedifference between (i) the smoothed positive phase signal and (ii) thesmoothed negative phase signal. An output voltage Verr of the erroramplifier 12 j is expressed as a2×(<V2 p>−<V2 n>)+Vref, which is the sumof (i) a reference voltage Vref and (ii) the product a2×(<V2 p>−<V2 n>)obtained by multiplying an offset voltage Vos=<V2 p>−<V2 n> by a gain a2of the error amplifier 12 j. Here, the reference voltage Vref is anoutput voltage of the reference voltage source 15 which is connected toa negative input terminal of the error amplifier 12 j. The outputvoltage Verr (hereinafter, referred to as “error amplifier output”) ofthe error amplifier 12 j is supplied into the MCU 13.

The LOS detection circuit 12 k identifies a section in which a value ofa received optical signal is an off level, i.e., a section in which alight current Ipd flowing through the photodetector element 11 hasbecome equal to or less than a predetermined threshold value (such asection is hereinafter referred to as a “no-signal section”). Thepredetermined threshold value is set to be, for example, (i)approximately equal to a level of a dark current flowing through thephotodetector element 11, or (ii) approximately equal to a photoelectriccurrent flowing through the photodetector element 11 when weak light isreceived. The LOS detection circuit 12 k further generates an LOS signalindicating an identified no-signal section. At the same time, the LOSdetection circuit 12 k stores a value of the LOS signal thus generatedin a register (not illustrated). In the present embodiment, the LOSsignal is a digital signal whose value is (i) “1” in a no-signal sectionand (ii) “0” outside of a no-signal section. Furthermore, in the presentembodiment, the register in which a value of the LOS signal is stored isa clear-on-read register, which updates the value after the value hasbeen read out. This means that, in a first readout after a no-signalsection has ended, “1” is read out as the value of the LOS signal.Readout of “0” as a value of the LOS signal is limited to a case where anon-LOS state continues from a previous readout to the current readout.The value of the LOS signal stored in this register is read out by theMCU 13 through the I2C interface 12 m.

The present embodiment employs a configuration, as illustrated in FIG.1, in which a resistor R connected in series to the photodetectorelement 11 has a dropped voltage R×Ipd and this dropped voltage issupplied to the LOS detection circuit 12 k through an operationalamplifier OP. The LOS detection circuit 12 k identifies a no-signalsection by referring to an output voltage of the operational amplifierOP.

Note that with the exception of the error amplifier 12 j and the I2Cinterface 12 m, the receiving circuit 12 includes 2 or more sets of theabovementioned components adequate for 2 or more channels (in thepresent embodiment, adequate for 4 channels). The switch 12 i switches achannel serving as a signal source of a differential signal that is tobe supplied to the error amplifier 12 j. In other words, thedifferential amplifier 12 e outputs a differential signal of a channelselected by the switch 12 i, and the differential signal thus outputtedis subsequently supplied to the error amplifier 12 j.

The MCU 13 is a component for executing offset cancellation processingand includes, as illustrated in FIG. 1, a voltage readout section 13 a;an LOS readout section 13 b; a threshold voltage changing section 13 c;a channel switching section 13 d; and an I2C interface 13 e. Here,“offset cancellation processing” refers to processing done to bring theoffset voltage Vos closer to 0 [V], i.e., processing that brings theerror amplifier 12 j output voltage Verr closer to the reference voltageVref. This processing is done by changing a level of the thresholdvoltage Vth generated by the variable voltage source 12 b. Note that anexplanation of (i) a flow of the offset cancellation processing and (ii)timing of execution of the offset cancellation processing is providedlater in reference to a different drawing.

The nonvolatile memory 14 is a component for recording a level of thethreshold voltage Vth (more precisely, a numerical value expressing thelevel of the threshold voltage Vth). In the present embodiment, anEEPROM (registered trademark) is used as the nonvolatile memory 14. TheMCU 13 executes threshold voltage write processing in which a level ofthe threshold voltage Vth is written to the nonvolatile memory 14. Thisprocessing is executed when operation of the MCU 13 ends (immediatelyprior to when operation of the MCU 13 ends), for example, at the time ofpower-off. The MCU 13 further executes threshold voltage readoutprocessing in which the level of the threshold voltage Vth is read outfrom the nonvolatile memory 14. This processing is executed whenoperation of the MCU 13 commences (immediately after operation of theMCU 13 commences), for example, at the time of power-on. The level ofthe threshold voltage Vth read out from the nonvolatile memory 14 at thetime of commencement of the operation (the level of the thresholdvoltage Vth written to the nonvolatile memory 14 at the time when thelast time operation ended) is used by the MCU 13 as an initial value ofthe threshold voltage Vth in the offset cancellation processing.

In a case where the threshold voltage write processing is executed atthe time of power-off, for example, an alarm set off by a power supplymonitor IC (not illustrated) may serve as a trigger to commence thethreshold voltage write processing, the alarm being set off when a powersupply voltage becomes less than a predetermined threshold value. Asillustrated in FIG. 1, if a voltage holding circuit including acondenser C1 and a diode D1 is inserted between a power supply and theMCU 13, and, at the same time, another voltage holding circuit includinga condenser C2 and a diode D2 is inserted between another power supplyand the nonvolatile memory 14, then it becomes possible to make the MCU13 and the nonvolatile memory 14 operate normally even after a level ofthe power supply voltage becomes less than a predetermined thresholdvalue.

[Flow of Offset Cancellation Processing]

The MCU 13 executes the offset cancellation processing in accordancewith the flowchart of FIG. 2. FIG. 2 is a flowchart showing a flow ofthe offset cancellation processing executed by the MCU 13. The followingdescribes operation of the MCU 13 during each step of the flowchart ofFIG. 2.

Step S11 (voltage readout processing): The MCU 13 reads out, through anA/D converter, (i) a voltage Verr outputted by the error amplifier 12 jand (ii) a reference voltage Vref outputted by the reference voltagesource 15. After this, the MCU 13 proceeds to step S12 of the offsetcancellation processing. The voltage readout section 13 a of the MCU 13illustrated in FIG. 1 is a section for executing this voltage readoutprocessing.

Step S12 (LOS readout processing): The MCU 13 reads out, through the I2Cinterface 13 e, a value of an LOS signal generated by the LOS detectioncircuit 12 k. After this, the MCU 13 proceeds to step S13 of the offsetcancellation processing. The LOS readout section 13 b of the MCU 13illustrated in FIG. 1 is a section for performing this LOS readout step.

Step S13 (LOS determination processing): The MCU 13 determines whetheror not the value of the LOS signal read out by the LOS detection circuit12 k is 0. In a case where the value of the LOS signal is 0, the MCU 13proceeds to step S14 of the offset cancellation processing. However, ina case where the value of the LOS signal is 1, the MCU 13 ends theoffset cancellation processing.

Step S14 (threshold voltage change processing): The MCU 13 changes alevel of the threshold voltage Vth generated by the variable voltagesource 12 b. The MCU 13 performs this change based on respective valuesof the reference voltage Vref and an error amplifier output Verr, therespective values both being read out by the voltage readout section 13a. After this, the MCU 13 ends the offset cancellation processing. Thethreshold voltage changing section 13 c of the MCU 13 illustrated inFIG. 1 is a section for executing this threshold voltage changeprocessing. When Verr>Vref+Δ, the threshold voltage changing section 13c sets a value of a variable expressing the threshold voltage Vth to avalue greater by δ than the present value. In contrast, whenVerr<Vref−Δ, the threshold voltage changing section 13 c sets the valueof the variable expressing the threshold voltage Vth to be less by δthan the present value. Here, Δ is a positive constant, predetermined asa tolerance. The threshold voltage changing section 13 c furthercontrols the variable voltage source 12 b, through the I2C interface 13e, such that the level of the threshold voltage Vth matches the setvalue. Note that an explanation of a flow of the threshold voltagechange processing is provided later in reference to a different drawing.

The following describes why it is possible to bring the offset voltageVos closer to 0 [V] via the above offset cancellation processing. In acase where Vos>Δ, (equivalent to: Verr>Vref+Δ), i.e., in a case wherethe threshold voltage Vth is too low with respect to an average level(an average value of a high level and a low level) of a voltage signaloutputted from the transimpedance amplifier 12 a, the threshold voltageVth is increased by the threshold voltage change processing. Thisresults in a reduction of a value of the offset voltage Vos. Conversely,in a case where Vos<−Δ, (equivalent to: Verr<Vref−Δ), i.e., in a casewhere the threshold voltage Vth is too high with respect to the averagelevel of the voltage signal outputted from the transimpedance amplifier12 a, the threshold voltage Vth is reduced by the threshold voltagechange processing. This results in an increase of the value of theoffset voltage Vos. Therefore, repetition of the above offsetcancellation processing makes it possible to make an absolute value|Vos| of the offset voltage Vos equal to or less than the tolerance Δ.

However, if the above threshold voltage change processing is repeated ina no-signal section in which a received optical signal continues to bean off-level value, then the level of the threshold voltage Vth willdecrease limitlessly. This may greatly distort a waveform of adifferential signal immediately after the start of a signal sectionfollowing a no-signal section. In view of this, the above offsetcancellation processing employs a configuration in which the thresholdvoltage change processing is skipped in a no-signal section. Thisprevents a waveform of a differential signal from being distortedgreatly immediately after the start of a signal section following ano-signal section.

Note that in the above offset cancellation processing, there are twoexecution modes, each mode having a different speed at which thethreshold voltage Vth is changed. Hereinafter, an execution mode inwhich the threshold voltage Vth is changed at a slower speed, i.e., asecond execution mode, is referred to as a “normal control mode.”Conversely, an execution mode in which the threshold voltage Vth ischanged at a faster speed, i.e., a first execution mode, is referred toas an “accelerated control mode.” Either one of the execution modes ofthe offset cancellation processing is set independently for eachchannel.

The MCU 13 uses a 1-bit (2-step) binary number as a variable expressingthe execution mode of the offset cancellation processing for eachchannel. Hereinafter, this variable is referred to as an “acceleratedcontrol flag ACE”. The MCU 13 further uses an 8-bit (256-step) binarynumber as a variable expressing the threshold voltage Vth. Hereinafter,this variable is referred to as “variable Vth”.

When a value of the accelerated control flag ACF is 1, i.e., when theoffset cancellation processing is executed in the accelerated controlmode, the MCU 13 changes, by 2 steps ( 1/128 of an entire variablerange, corresponding to an amount of change δ=0x02), a value of thevariable Vth each time step S14 (threshold voltage change processing) isperformed. Conversely, when the value of the accelerated control flagACF is 0, i.e., when the offset cancellation processing is executed inthe normal control mode, the MCU 13 changes, by 1 step ( 1/256 of theentire variable range) (by an amount of change δ=0x01), the value of thevariable Vth each time step S14 (threshold voltage change processing) isperformed.

[Timing of Execution of Offset Cancellation Processing]

The MCU 13 determines timing of execution of the offset cancellationprocessing in accordance with the flowchart of FIG. 3. FIG. 3 is aflowchart showing a flow of processing, in its entirety, executed by theMCU 13. The following describes operation of the MCU 13 during each stepof the flowchart of FIG. 3.

Step S101: The MCU 13 initializes the value of the accelerated controlflag ACF of every channel to 1. The MCU 13 further initializes a valueof a main counter n to 1.

Step S102: The MCU 13 repeatedly determines whether or not a 1millisecond timer counter has counted up. This is repeated until aresult of this determination becomes “yes”. Once the result is “yes”,the MCU 13 proceeds to step S103.

Step S103: The MCU 13 determines whether or not the value of the maincounter is one of the following: 1, 2, 3, and 4. If a result of thisdetermination is “yes”, the MCU 13 then proceeds to step S104. If theresult is “no”, the MCU 13 then proceeds to step S111 instead.

Step S104: The MCU 13 determines whether or not a value of anaccelerated control flag ACF of a channel subject to the offsetcancellation processing is 0. If a result of this determination is“yes”, the MCU 13 then proceeds to step S105. If the result is “no”, theMCU 13 then proceeds to step S106 instead.

Step S105: The MCU 13 initializes a value of a sub-counter m to 1. Atthe same time, the MCU 13 sets an upper limit value M of the value ofthe sub-counter m to 1. The MCU 13 then proceeds to step S107.

Step S106: The MCU 13 initializes the value of the sub-counter m to 1.At the same time, the MCU 13 sets the upper limit value M of the valueof the sub-counter m to 2. The MCU 13 then proceeds to step S107.

Step S107: The MCU 13 executes the offset cancellation processing asshown in FIG. 2. The MCU 13 then proceeds to step S108.

Step S108: The MCU 13 increases, by one increment, the value of thesub-counter m, then proceeds to step S109.

Step S109: The MCU 13 determines whether or not the value of thesub-counter m has reached the upper-limit value. If a result of thisdetermination is “yes”, the MCU 13 then proceeds to step S110. If theresult is “no”, then the MCU 13 returns to step S107 instead.

Step S110: The MCU 13 switches the channel subject to the offsetcancellation processing to a next channel. The MCU 13 then proceeds tostep S112. Note that if the channel subject to the processing is achannel k (k=1, 2, or 3), then after the above switching, the channelsubject to the processing becomes a channel k+1. If the channel subjectto the processing is channel 4, then after the above switching, thechannel subject to the processing becomes channel 1. The channelswitching section 13 d of the MCU 13 illustrated in FIG. 1 is a sectionfor executing this channel switching processing. The channel switchingsection 13 d controls the switch 12 i through the I2C interface 13 esuch that a differential signal outputted from the differentialamplifier 12 e of the channel subject to the offset cancellationprocessing (i.e., subject to the offset cancellation processing afterchannel switching) is supplied to the error amplifier 12 j.

Step S111: The MCU 13 executes some other processing (optionalprocessing other than the offset cancellation processing), then proceedsto step S112. Note that this step is provided to avoid a situation inwhich the MCU 13 is dedicated solely to the offset cancellationprocessing.

Step S112: The MCU 13 increases, by one increment, the value of the maincounter n, then proceeds to step S113.

Step S113: The MCU 13 determines whether or not the value of theaccelerated control flag ACF of every channel is 0. If a result of thisdetermination is “yes”, the MCU 13 then proceeds to step S114. If theresult is “no”, i.e., if the value of the accelerated control flag ACFof any one of the channels is 1, the MCU 13 then proceeds to step S116.

Steps S114 and S115: The MCU 13 determines whether or not the value ofthe main counter n is greater than 100. If a result of thisdetermination is “yes”, then the MCU 13 resets the value of the maincounter n to 1 and returns to step S102.

Steps S116 and S117: The MCU 13 determines whether or not the value ofthe main counter n is greater than 5. If a result of this determinationis “yes”, then the MCU 13 resets the value of the main counter n to 1and returns to step S102.

In a case where the timing of execution of the offset cancellationprocessing is determined in accordance with the flowchart of FIG. 3, anexecution form of the offset cancellation processing becomes any one ofExecution Forms 1 through 3 shown in the table below.

TABLE 1 ACF = 1 in channel ACF = 0 in channel subject to processingsubject to processing ACF = 1 <Execution Form 1> <Execution Form 2> inany one channel Amount of change δ: Amount of change δ: 2 steps 1 stepExecution frequency: Execution frequency: 2 times/5 msec 1 time/5 msecACF = 0 <Execution Form 3> in every channel Amount of change δ: 1 stepExecution frequency: 1 time/100 msec

At the time of commencement of operation (at the time of power-on), theexecution mode of the offset cancellation processing is set to be theaccelerated control mode for every channel (step S101). Therefore, theexecution form of the offset cancellation processing for each channelbecomes <Execution Form 1> shown in Table 1.

If the execution mode of the offset cancellation processing for any oneof the channels is subsequently switched from the accelerated controlmode to the normal control mode (“Yes” in step S104), then the executionform of the offset cancellation processing for that channel transitionsfrom <Execution Form 1> to <Execution Form 2> shown in Table 1. Such atransition from <Execution Form 1> to <Execution Form 2> occursindependently for each channel.

If the execution mode of the offset cancellation processing for everychannel is subsequently switched from the accelerated control mode tothe normal control mode (“Yes” in step S113), then the execution form ofthe offset cancellation processing for each channel transitions from<Execution Form 2> to <Execution Form 3> shown in Table 1. Thetransition from <Execution Form 2> to <Execution Form 3> occurssimultaneously in all the channels. Note that in the present embodiment,a frequency of execution of the threshold value change processing in<Execution Form 3> is once per 100 milliseconds, but the frequency isnot limited to this. For example, the frequency of execution of thethreshold value change processing in <Execution Form 3> mayalternatively be once per 5 seconds. In such a case, in theabovementioned step S114, the MCU 13 determines whether or not the valueof the main counter n is greater than 5,000.

One example of operation of the MCU 13 is shown in FIG. 4. Thisoperation is realized by determining the timing of execution of theoffset cancellation processing in accordance with the flowchart of FIG.3. (a) of FIG. 4 is a waveform chart depicting respective waveforms ofan optical signal and of an LOS signal. (b) of FIG. 4 shows an exampleof operation of the MCU 13 in section T1 shown in (a) of FIG. 4. (c) ofFIG. 4 shows an example of operation of the MCU 13 in section T2 shownin (a) of FIG. 4. (d) of FIG. 4 shows an example of operation of the MCU13 in section Tn shown in (a) of FIG. 4.

In communications in conformance with SAS 2.0, after power-on, a link-upsequence involving sending and receiving of a COMINIT signal, a COMSASsignal, an SNT (Speed Negotiation Transmit) signal, and an MTT (MaximumTransmitter Training) signal is repeated as shown in (a) of FIG. 4, forexample, five times at the maximum. Here, the COMINIT signal, the COMSASsignal, and the SNT signal are each an OOB signal in which a DATAsection and an IDLE section appear alternately (i.e., are each anexample of a “first optical signal” recited in the claims below). Eachof these OOB signals has significance as a signal not in terms of a bitpattern of the DATA section, but rather in terms of a width (duration)of the DATA section and the IDLE section. For example, the COMSAS signalis specified as being an OOB signal having a DATA section of 106nanoseconds and an IDLE section of 960 nanoseconds, the DATA section andthe IDLE section appearing alternately. The optical receiver 1 cantherefore correctly recognize these OOB signals regardless of whetheroffset cancellation has been completed or not. Conversely, the MTTsignal (an example of a “second optical signal” recited in the claimsbelow) is a data signal whose DATA section has a duration of 19.9milliseconds. The MTT signal has significance as a signal not in termsof a width of the DATA section, but rather in terms of a bit pattern ofthe DATA section. Due to this fact, in order for the optical receiver 1to correctly recognize an MTT signal, it is necessary to (i) completethe offset cancellation processing before receiving the MTT signal and(ii) accurately read the value of each component bit of the MTT signal.As shown in (a) of FIG. 4, an LOS signal read out by the MCU 13 has avalue of 0 in sections T1, T2, etc. (sections in which the MTT signal isreceived), as well as in sections Tn (sections in which some other datasignal is received). The threshold voltage change processing is executedfor such sections in which the LOS signal has a value of 0.

The MCU 13 reads out the LOS signal at a cycle which is set to be (i)longer than a DATA section of an OOB signal (the COMINIT signal, theCOMSAS signal, or the SNT signal; in the case of the COMSAS signal, theDATA section lasts 106 nanoseconds) and (ii) shorter than the DATAsection (19.9 milliseconds) of the MTT signal. Thus, during reception ofthe OOB signal, readout of the LOS signal is performed once at most.During reception of the MTT signal, readout of the LOS signal isperformed at least twice (four times in the present embodiment). Notethat as shown in (a) of FIG. 4, in the DATA section of (i) the COMINITsignal, (ii) the COMSAS signal, and (iii) the SNT signal, the value ofthe LOS signal read out by the MCU 13 is not 0. This is because thereadout, by the MCU 13, of the value of the LOS signal is performedthrough the clear-on-read register. This means that the thresholdvoltage change processing is not performed for respective DATA sectionsof (i) a COMINIT signal, (ii) a COMSAS signal, and (iii) an SNT signal.Thus, even if the mark rate of a COMINIT signal, a COMSAS signal or anSNT signal is not 50%, the variable Vth will not be set to an incorrectvalue in a section in which one of the above signals is received. Notethat if the mark rate of a COMINIT signal, a COMSAS signal, or an SNTsignal is 50%, the threshold voltage change processing may be performedfor sections in which one of the above signals is received. In such acase, it is not necessary to perform readout of the value of the LOSsignal through the clear-on-read register.

(b) through (d) of FIG. 4 show timing of execution of the offsetcancellation processing for each channel. In each of (b) through (d) ofFIG. 4, the timing is indicated by placing, along an axis representingtime, rectangles representing periods of execution of the offsetcancellation processing. In (b) through (d) of FIG. 4, the offsetcancellation processing represented by white rectangles is unaccompaniedby the threshold voltage change processing; the offset cancellationprocessing represented by grey (dotted) rectangles is accompanied by thethreshold voltage change processing in which the value of the variableVth is changed by 1 step (0x01); and the offset cancellation processingrepresented by black rectangles is accompanied by the threshold voltagechange processing in which the value of the variable Vth is changed by 2steps (0x02).

(b) of FIG. 4 shows a typical example of operation of the MCU 13 duringa period of time in which the offset cancellation processing is executedin the accelerated control mode for every channel. In an example shownin (b) of FIG. 4, for every channel, the offset cancellation processingis executed twice per 5 milliseconds, the offset cancellation processingbeing accompanied by the threshold voltage change processing in whichthe value of the variable Vth is changed by 2 steps (“Execution Form 1”in Table 1). Note, however, that the offset cancellation processingimmediately after fall of the LOS signal is not accompanied by thethreshold voltage change processing. This is because readout of the LOSsignal is performed through the clear-on-read register. This makes itpossible, in section Ti in which the optical receiver 1 receives an MTTsignal, for the MCU 13 to (i) execute the threshold voltage changeprocessing 7 times at the maximum for each channel, and (ii) change thevalue of the variable Vth by a maximum of 14 steps (0x0e) for eachchannel.

Thereafter, the MCU 13 switches the execution mode of the offsetcancellation processing from the accelerated control mode to the normalcontrol mode. The order in which this switching is performed for thechannels follows the order in which offset cancellation is performed(the order in which the absolute value |Vos| of the offset voltage Vosbecomes equal to or less than the tolerance Δ). (c) of FIG. 4 shows atypical example of operation of the MCU 13 during a period in whichthere is a mix of (i) a channel(s) for which the offset cancellationprocessing is being executed in the accelerated control mode and (ii) achannel(s) for which the offset cancellation processing is beingexecuted in the normal control mode. In the example shown in (c) of FIG.4, for both channel 2 and channel 4, the offset cancellation processingis executed twice per 5 milliseconds, the offset cancellation processingbeing accompanied by the threshold voltage change processing in whichthe value of the variable Vth is changed by 2 steps (“Execution Form 1”in Table 1). With regards to channel 1, for the period of time from 0milliseconds after starting to 5 milliseconds after starting, the offsetcancellation processing is performed twice per 5 milliseconds, theoffset cancellation processing being accompanied by the thresholdvoltage change processing in which the value of the variable Vth ischanged by 2 steps (“Execution Form 1” in Table 1). Further, for channel1, for the period of time from 5 milliseconds after starting to 20milliseconds after starting, the offset cancellation processing isexecuted once per 5 milliseconds, the offset cancellation processingbeing accompanied by the threshold voltage change processing in whichthe value of the variable Vth is changed by 1 step (“Execution Form 2”in Table 1). With regards to channel 3, for the period of time from 0milliseconds after starting to 10 milliseconds after starting, theoffset cancellation processing is performed twice per 5 milliseconds,the offset cancellation processing being accompanied by the thresholdvoltage change processing in which the value of the variable Vth ischanged by 2 steps (“Execution Form 1” in Table 1). Further, for channel3, for the period of time from 10 milliseconds after starting to 20milliseconds after starting, the offset cancellation processing isperformed once per 5 milliseconds, the offset cancellation processingbeing accompanied by the threshold voltage change processing in whichthe variable Vth is changed by 1 step (“Execution Form 2” in Table 1).Note that (c) of FIG. 4 depicts a state immediately after the LOS signalhas fallen and that the first offset cancellation processing executed isnot accompanied by the threshold voltage change processing.

(d) of FIG. 4 shows a typical example of operation of the MCU 13 duringa period of time in which the offset cancellation processing is executedin the normal control mode for every channel. In the example shown in(d) of FIG. 4, for every channel, the offset cancellation processing isexecuted once per 100 milliseconds, the offset cancellation processingbeing accompanied by the threshold voltage change processing in whichthe value of the variable Vth is changed by 1 step (“Execution Form 3”in Table 1). Note that (d) of FIG. 4 depicts a state immediately afterfall of the LOS signal. In this state, the first offset cancellationprocessing performed is not accompanied by the threshold voltage changeprocessing.

Thus, the MCU 13 can change the value of the variable Vth by a maximumof 14 steps (0x0e) in the section Ti in which the optical receiver 1receives an MTT signal. Thus, in sections T1 through T4, in which theoptical receiver 1 receives respective first through fourth MTT signals,the MCU 13 can change the value of the variable Vth by a maximum of 56(=14×4) steps. In a case where an optical transmitter includes a lightemitting element that is a VCSEL, a VCSEL temperature change causesfluctuation in received power. This power fluctuation causes offset. Anamount of change in variable Vth required to cancel this offset (i.e.,to bring the absolute value |Vos| of the offset voltage Vos equal to orless than the tolerance Δ) is 56 steps or fewer. The reason for this isexplained later. This makes it possible for the MCU 13 to reliablycomplete cancellation of the offset in sections T1 through T4, in whichthe optical receiver 1 receives the first through fourth MTT signals,respectively. This in turn allows the MCU 13 to correctly read the bitpattern of at least a fifth MTT signal and establish a link before fivelink-up sequences end.

[Amount of Change in Variable Vth Required to Cancel Offset]

In a case where an optical transmitter includes a light emitting elementthat is a VCSEL, a VCSEL temperature change causes fluctuation inreceived power. This power fluctuation causes offset. An amount ofchange in variable Vth required to cancel this offset (i.e., to bringthe absolute value |Vos| of the offset voltage Vos equal to or less thanthe tolerance Δ) is, as mentioned above, 56 steps or fewer. Thefollowing discusses this in more detail with reference to FIG. 5.

Firstly, in a case where VCSEL deterioration over time (explained later)is not considered, a range of fluctuation of the power of an opticalsignal received by the optical receiver 1 (this power is hereinafterreferred to as “received power”) is, for example, −2.3 dBm/+2.6 dBm. Thevariable Vth has an initial value which is determined such that offsetfor any given received power within the above range of fluctuation iscancelled. The maximum value that the variable Vth can take on as theinitial value is determined such that offset for a received power of+2.6 dBm is cancelled. Likewise, the minimum value that the variable Vthcan take on as the initial value is determined such that offset for areceived power of −2.3 dBm is cancelled.

FIG. 5 is a graph obtained by confirming, via experiment, therelationship between (i) received power (in dBm) and (ii) the amount ofchange in variable Vth (in steps) necessary to cancel offset for thereceived power. This graph shows cases where the initial value of thevariable Vth is set to a maximum value and a minimum value,respectively. The graph of FIG. 5 shows, for example, that if thereceived power is +1.5 dBm in a case where the initial value of thevariable Vth is set to be a maximum value, offset can be cancelled byincreasing the value of the variable Vth by 30 steps. The graph of FIG.5 also shows, for example, that alternatively, if the actual receivedpower is −1.5 dBm in a case where the initial value of the variable Vthis set to be a minimum value, offset can be cancelled by reducing thevalue of the variable Vth by 10 steps.

The fluctuation in received power caused by the VCSEL temperature changeis in a range of 2.0 dB (−1.5 dB/+0.5 dB). The amount of change invariable Vth required to cancel offset caused by such fluctuation inreceived power is 56 steps or fewer. In actuality, as shown in FIG. 5,in a case where the initial value of the variable Vth is set to be amaximum value, the received power may decrease, due to the VCSELtemperature change, to as low as +0.6 dBm (=+2.6 dBm−2.0 dBm). However,offset occurring in such a case can be cancelled by increasing the valueof the variable Vth by 50 steps. As shown in FIG. 5, in a case where theinitial value of the variable Vth is set to be a minimum value, thereceived power may increase, due to the VCSEL temperature change, to ashigh as −0.3 (=−2.3+2.0) dBm. However, offset occurring in such a casecan be cancelled by decreasing the value of the variable Vth by 25steps.

In the present embodiment, the amount of change in variable Vth duringthe offset cancellation processing, in Execution Form 1, is 2 steps. Itshould be noted that this amount of change is set in response to thefact that the maximum amount of change in variable Vth required tocancel the offset due to the VCSEL temperature change is 50 steps. Thereason for this is as follows. In a case where the amount of change invariable Vth during the offset cancellation processing is set to be 1step instead in the abovementioned Execution Form 1, the maximum amountof change in variable Vth in sections T1 through T4, i.e., in sectionsin which the optical receiver 1 receives the first through fourth MTTsignals, respectively, is 28 (=7×4) steps. This amount is less than 50steps, which is the amount of change in Vth required to cancel theoffset due to the VCSEL temperature change.

[Nonvolatile Storage of Value of Variable Vth]

The power of an optical signal received by the optical receiver 1 (thispower is hereinafter referred to as “received power”) fluctuates due toVCSEL temperature change and VCSEL deterioration over time, the VCSELbeing provided as a light emitting element of an optical transmitter.The range of fluctuation in received power due to the VCSEL temperaturechange is, as mentioned above, approximately −1.5 dB/+0.5 dB, whereas arange of fluctuation in received power due to the VCSEL deteriorationover time is approximately −2 dB/+0 dB.

In the optical receiver 1 according to the present embodiment, it ispossible to change the value of the variable Vth by a maximum of 56steps by the time when a final MTT signal is received, the variable Vthbeing represented by an 8-bit binary number. Therefore, even if theVCSEL temperature change causes the received power to fluctuate in arange of approximately 2.0 dB (−1.5 dB/+0.5 dB), it is possible toreliably cancel offset before receiving an MTT signal in a final link-upsequence unless the received power fluctuates due to the VCSELdeterioration over time.

However, in a case where the fluctuation in received power due totemperature change of the light emitting element coincides withfluctuation in received power due to deterioration over time of thelight emitting element, a range of resulting fluctuation in receivedpower becomes −3.5 dB/+0.5 dB (a fluctuation over a range of 4 dB). Theamount of change in variable Vth required to cancel offset produced bysuch a fluctuation in received power is more than 56 steps. Inactuality, as shown in FIG. 5, in a case where the initial value of Vthis set to a maximum value, the amount of change in variable Vth requiredto cancel offset is 80 steps or more. Therefore, if the initial value ofthe variable Vth is fixed to be a value set at the time of shipment fromfactory, then it becomes impossible to cancel offset before reception ofthe MTT signal in the final link-up sequence.

In order to address this issue, the optical receiver 1 includes aconfiguration in which (i) at the time when operation ends, the value ofthe variable Vth at that point is written to the nonvolatile memory 14and (ii) the value of the variable Vth read out from the nonvolatilememory 14 at the time when operation commences is used as the initialvalue in the offset cancellation processing. With this configuration,even if (i) fluctuation in received power due to temperature change ofthe light emitting element coincides with fluctuation in received powerdue to deterioration over time of the light emitting element, and (ii)the received power fluctuates in a range of approximately −3.5 dB/+0.5dB, it is possible to reduce the amount of change in variable Vth (theamount of change from the initial value) required to cancel offset toapproximately −1.5 dB/+0.5 dB (the range of the fluctuation in receivedpower due to the temperature change). Thus, it is possible to ensurethat offset is cancelled before reception of the MTT signal in the finallink-up sequence. In other words, the optical receiver 1 avoids problemssuch as (i) uncompleted offset cancellation at the end of the fourthlink-up sequence or (ii) reduced precision of offset cancellation. Notethat even if deterioration over time of the light emitting elementoccurs during operation of the optical receiver 1, the variable Vthchanges while the offset cancellation processing is being executed. Inthis case, the variable Vth changes by following gradual fluctuation inreceived power that accompanies the deterioration over time of the lightemitting element. Therefore, no problems will occur even ifdeterioration over time of the light emitting element occurs duringoperation of the optical receiver 1.

Note that although the present embodiment includes a configuration inwhich the threshold voltage write processing (processing in which thevalue of the variable Vth is written to the nonvolatile memory 14) isexecuted at the time when operation of the MCU 13 ends, the presentembodiment is not limited to this configuration. For example, in analternative configuration, the threshold voltage write processing may beperformed periodically during operation of the MCU 13. Employing such aconfiguration would eliminate the need to provide the voltage holdingcircuit as described above.

Note that if an execution cycle of the threshold voltage writeprocessing is lengthened, the nonvolatile memory 14 has a longer life,but the variable Vth in nonvolatile storage will be less accurate as aninitial value. This loss of accuracy is caused by an increase in lengthof time between (i) when the MCU 13 last performs the threshold voltagewrite processing and (ii) when operation of the MCU 13 ends. Incontrast, if the execution cycle of the threshold voltage writeprocessing is shortened, the nonvolatile memory 14 has a shorter life,but the variable Vth in nonvolatile storage becomes more accurate as aninitial value. This increase in accuracy is caused by a decrease inlength of time between (i) when the MCU 13 last performs thresholdvoltage write processing and (ii) when operation of the MCU 13 ends. Inview of both the life of the nonvolatile memory 14 and the accuracy ofthe variable Vth in nonvolatile storage as an initial value, it ispreferable that the execution cycle of the threshold voltage writeprocessing be not less than 30 minutes and not more than 1 hour and 30minutes. It is more preferable that the execution cycle of the thresholdvoltage write processing be 1 hour.

[Threshold Voltage Change Processing]

The MCU 13 executes threshold voltage change processing in accordancewith the flowchart of FIG. 6. FIG. 6 is a flowchart showing a flow ofthreshold voltage change processing. The following describes operationof the MCU 13 during each step of the flowchart of FIG. 6.

Note that this threshold voltage change processing makes use of acontrol direction flag CDF in addition to the accelerated control flagACF. The control direction flag CDF is a flag which takes on one of 3possible values. The control direction flag CDF takes on a value of 1 inthe case of accelerated control whose direction is a direction thatincreases the value of the variable Vth. The control direction flag CDFtakes on a value of 2 in the case of accelerated control whose directionis a direction that decreases the value of the variable Vth. The controldirection flag CDF takes on an initial value of 0 when (i) acceleratedcontrol changes in direction, or (ii) an error amplifier output Verr iswithin a range equaling a reference voltage Vref±a tolerance Δ.

Step S201: The MCU 13 determines whether or not the error amplifieroutput Verr has a value greater than a value of the sum of (i) a valueof the reference voltage Vref and (ii) the tolerance Δ. If a result ofthis determination result is “yes”, the MCU 13 then proceeds to stepS202. If the result is “no”, the MCU 13 then proceeds to step S209.

Step S202: The MCU 13 determines whether or not the control directionflag CDF has a value of“2”. If a result of this determination is “yes”,the MCU 13 then proceeds to step S203. If the result is “no”, the MCU 13then proceeds to step S205.

Step S203: The MCU 13 sets the value of the accelerated control flag ACFto 0, and then proceeds to step S204.

Step S204: The MCU 13 sets the value of the control direction flag CDFto 0, and then proceeds to step S205.

Step S205: The MCU 13 determines whether or not the value of theaccelerated control flag ACF is 1. If a result of this determination is“yes”, the MCU 13 then proceeds to step S206. If the result is “no”, theMCU 13 then proceeds to step S208.

Step S206: The MCU 13 increases the value of variable Vth by 2 steps(0x02), and then proceeds to step S207.

Step S207: The MCU 13 sets the value of the control direction flag CDFto 1, and then proceeds to step S220.

Step S208: The MCU 13 increases the value of variable Vth by 1 step(0x01), and then proceeds to step S220.

Step S209: The MCU 13 determines whether or not the error amplifieroutput Verr has a value less than a value obtained by subtracting thetolerance Δ from the value of the reference voltage Vref. If a result ofthis determination is “yes”, the MCU 13 then proceeds to step S210. Ifthe result is “no”, the MCU 13 then proceeds to step S217.

Step S210: The MCU 13 determines whether or not the control directionflag CDF has a value of 1. If a result of this determination is “yes”,the MCU 13 then proceeds to step S211. If the result is “no”, the MCU 13then proceeds to step S213.

Step S211: The MCU 13 sets the value of the accelerated control flag ACFto 0, and then proceeds to step S212.

Step S212: The MCU 13 sets the value of the control direction flag CDFto 0, and then proceeds to step S213.

Step S213: The MCU 13 determines whether or not the value of theaccelerated control flag ACF is 1. If a result of this determination is“yes”, the MCU 13 then proceeds to step S214. If the result is “no”, theMCU 13 then proceeds to step S216.

Step S214: The MCU 13 decreases the value of the variable Vth by 2 steps(0x02), and then proceeds to step S215.

Step S215: The MCU 13 sets the value of the control direction flag CDFto 2, and then proceeds to step S220.

Step S216: The MCU 13 decreases the value of variable Vth by 1 step(0x01), and then proceeds to step S220.

Step S217: The MCU 13 determines whether or not the value of theaccelerated control flag ACF is 1. If a result of this determination is“yes”, the MCU 13 proceeds to step S218. If the result is “no”, the MCU13 ends the threshold voltage change processing.

Step S218: The MCU 13 sets the value of the accelerated control flag ACFto 0, and then proceeds to step S219.

Step S219: The MCU 13 sets the value of the control direction flag CDFto 0, and then ends the threshold voltage change processing.

Step S220: The MCU 13 controls the variable voltage source 12 b suchthat a level of the threshold voltage Vth matches the value of thevariable Vth obtained in step S206, S208, S214, or S216.

FIG. 7 shows an example of operation of the MCU 13, the operation beingrealized by execution of the threshold voltage change processing inaccordance with the flowchart of FIG. 6. The initial state of thisexample of operation is as follows: Verr>Vref+Δ; accelerated controlflag ACF=1; and control direction flag CDF=1.

(a) of FIG. 7 is a graph showing a change in amount of the erroramplifier output Verr over time. (b) of FIG. 7 is a graph showing achange in the value of the accelerated control flag ACF over time. (c)of FIG. 7 is a graph showing a change over time of the control directionflag CDF. (d) of FIG. 7 is a graph showing a change in level of thethreshold voltage Vth over time.

In a case where Verr>Vref+Δ (“Yes” in step S201); the control directionflag CDF=1 (“No” in step S202); and the accelerated control flag ACF=1(“Yes” in step S205), offset cancellation processing A is executedrepeatedly at a frequency of twice per 5 milliseconds. In the abovecase, the offset cancellation processing A is accompanied only byprocessing in which the value of the variable Vth is increased by 2steps (step S208). Through this processing, the level of the thresholdvoltage Vth gradually increases as shown in (d) of FIG. 7, and, as aresult, the level of the error amplifier output Verr gradually decreasesas shown in (a) of FIG. 7.

In a case where repeated execution of the offset cancellation processingA causes the error amplifier output Verr to be less than Vref−Δ (“No” instep S201 and “Yes” in step S209), i.e., in a case where repeatedexecution of the offset cancellation processing A causes sign inversionof the offset voltage Vos, the following are each executed: (i)processing in which respective values of the accelerated control flagACF and the control direction flag CDF are both changed to 0 (“Yes” instep S210; and steps S211 and S212); and (ii) one or two times of offsetcancellation processing B. Here, “offset cancellation processing B”refers to offset cancellation that is accompanied by processing in whichthe value of the variable Vth is decreased by 1 step (“No” in step S213;and step S216). In a case where the level of the error amplifier outputVerr is not within the range equaling Vref±Δ, the offset cancellationprocessing B is executed twice. In a case where the level of the erroramplifier output Verr is within the range equaling Vref±Δ, the offsetcancellation processing B is executed once. Such processing slightlydecreases the level of the threshold voltage Vth as shown in (d) of FIG.7, and, as a result, slightly increases the level of the error amplifieroutput Verr as shown in (a) of FIG. 7.

In a case where the level of the error amplifier output Verr is withinthe range equaling Vref±Δ as a result of execution of the offsetcancellation processing B (“No” in step S201; “No” in step S209; and“No” in step S217), the threshold voltage Vth is not changed. Thiscauses the level of the threshold voltage Vth to be kept constant asshown in (d) of FIG. 7, and, as a result, the level of the erroramplifier output Verr is kept constant as shown in (a) of FIG. 7.

[Active Optical Cable]

The optical receiver 1 according to the present embodiment can be usedas a connector of an active optical cable.

FIG. 8 is a block diagram illustrating a configuration of an activeoptical cable 100. As shown in FIG. 8, the active optical cable 100includes an optical cable 101 and a pair of connectors 102 and 103, eachconnector being respectively provided on both ends of the optical cable101. The optical cable 101 contains eight optical fibers 104 a and 104b.

The connector 102 includes four AC coupling condensers 105 a, atransmission circuit 106 a, and four laser diodes (LDs) 107 a. Thesecomponents of the connector 102 function as an optical transmitter that(i) converts, into an optical signal, a voltage signal externallysupplied, and (ii) transmits the optical signal. The connector 102further includes four photodiodes (PDs) 108 b, a receiving circuit 109b, and four AC coupling condensers 110 b. These components of theconnector 102 function as an optical receiver that (i) converts areceived optical signal into a voltage signal and (ii) outputs, outsidethe connector 102, the voltage signal.

The optical receiver 1 according to the present embodiment is made up ofthe PDs 108 b and the receiving circuit 109 b, along with an MCU 111included in the connecter 102. Therefore, even if there are fluctuationsin power of an optical signal sent from the connector 103, it ispossible to (i) bring closer to 0 [V] an offset voltage of adifferential signal amplified by the receiving circuit 109 b and (ii)keep the difference between the offset voltage and 0 [V] (a deviation ofthe offset voltage from 0 [V]) equal to or less than a predeterminedtolerance. Furthermore, the threshold voltage change processingaccordingly required is skipped in a no-signal section, in which thevalue of an optical signal sent from the connector 103 is an off level.This means that a waveform of the voltage signal outputted from theconnector 102 will not be distorted immediately after the start of asignal section following a no-signal section.

The connector 103 includes four photodiodes (PDs) 108 a, a receivingcircuit 109 a, and four AC coupling condensers 110 a. These componentsof the connector 103 function as an optical receiver that (i) converts areceived optical signal into a voltage signal and (ii) outputs thevoltage signal to an external device. The connector 103 further includesfour AC coupling condensers 105 b, a transmission circuit 106 b, andfour laser diodes (LDs) 107 b. These components of the connector 103function as an optical transmitter that (i) converts, into an opticalsignal, a voltage signal externally supplied and (ii) transmits theoptical signal.

The optical receiver 1 according to the present embodiment is made up ofthe PDs 108 a and the receiving circuit 109 a, along with an MCU 112provided in the connecter 103. Therefore, even if there are fluctuationsin the power of an optical signal sent from the connector 102, it ispossible to (i) bring closer to 0 [V] an offset voltage of adifferential signal amplified by the receiving circuit 109 a and (ii)keep the difference between the offset voltage and 0 [V] (a deviation ofthe offset voltage from 0 [V]) equal to or less than a predeterminedtolerance. Furthermore, the threshold voltage change processingaccordingly required is skipped in a no-signal section, in which thevalue of an optical signal sent from the connector 102 is an off level.This means that a waveform of the voltage signal outputted from theconnector 103 will not be distorted immediately after the start of asignal section following a no-signal section.

Thus, in the active optical cable 100, in a case where the connector 103transmits, to the connector 102, an optical signal having a signalsection that follows a no-signal section, a waveform of a voltage signaloutputted from the connector 102 will not be distorted immediately afterthe start of the signal section. Similarly, in a case where theconnector 102 transmits, to the connector 103, an optical signal havinga signal section that follows a no-signal section, a waveform of avoltage signal output from the connector 103 will not be distortedimmediately after the start of the signal section. Therefore, the activeoptical cable 100 can be used suitably in serial communications inconformance with standards in which it is necessary, in a link-upsequence, to send and receive an OOB signal, a signal including an EIsection, and the like. Examples of such standards encompass SAS 2.0 andPCIe 3.0.

In the active optical cable 100, an optical signal received by anoptical receiver (for example, PDs 108 a and receiving circuit 109 a) isan optical signal that is transmitted from a predetermined opticaltransmitter (for example, the transmission circuit 106 a and LDs 107 a)through a predetermined optical fiber (for example, optical fibers 104a). It is therefore possible to estimate, in advance, a range offluctuation of received power of the optical receiver. The estimate ismade based on temperature characteristics of light emitting elements(for example, LDs 107 a) that are components of the optical receiver.Because of this fact, in the active optical cable 100, it easy torealize offset cancellation processing that uses a predeterminedprogram.

It should be noted, however, that a range of applicability of thepresent invention is not limited to an active optical cable. The presentinvention can be applied in a light transceiver module, for example. Ina light transceiver module having a wide range of fluctuations (linkbudget) of received power, it is possible to increase the amount ofchange δ of the variable which expresses the threshold voltage Vth inthe accelerated control mode, such that the amount of change δ isgreater than that of the above embodiment. It is also possible toincrease the execution frequency of the offset cancellation processingso that the execution frequency will be greater than that of the aboveembodiment.

[Overview]

An optical receiver according to the present embodiment includes: aphotodetector element that converts an optical signal into an electriccurrent signal; a transimpedance amplifier that converts the electriccurrent signal into a voltage signal; a differential amplifier thatconverts the voltage signal into a differential signal, by performingdifferential amplification of a difference between the voltage signaland a threshold voltage; a no-signal detection circuit that detects ano-signal section of the optical signal; and a control section thatrepeatedly executes offset cancellation processing, the offsetcancellation processing including threshold voltage change processing inwhich the threshold voltage is changed such that an offset voltage ofthe differential signal is reduced, the control section skipping thethreshold voltage change processing in the no-signal section.

In other words, an optical receiver according to the present embodimentis an optical receiver including: a photodetector element that convertsan optical signal into an electric current signal; a transimpedanceamplifier that converts the electric current signal into a voltagesignal; a differential amplifier that converts the voltage signal into adifferential signal, by differential amplification of a differencebetween the voltage signal and a threshold voltage; a no-signaldetection circuit that detects a no-signal section of the opticalsignal; and a control section that repeatedly executes offsetcancellation processing, the offset cancellation processing, when beingperformed outside the above no-signal section, including thresholdvoltage change processing in which the threshold voltage is changed suchthat an offset voltage of the differential signal is reduced, and theoffset cancellation processing, when being performed in the no-signalsection, not including the threshold voltage change processing.

The above configuration makes it possible to cancel offset of thedifferential signal while causing no distortion of a waveform of avoltage signal immediately after the start of a signal section thatfollows a no-signal section.

It is preferable that the optical receiver according to the presentembodiment further include a variable voltage source that generates thethreshold voltage, the control section changing the threshold voltage bycontrolling the variable voltage source.

This configuration makes it possible to realize the control section byusing an electronic computer such as an MCU (Micro Controller Unit).

It is preferable that in the optical receiver according to the presentembodiment, the offset cancellation processing has two execution modeswhich are different from each other in an amount of change in thethreshold voltage during the threshold voltage change processing, thetwo execution modes including a first execution mode in which the amountof change in the threshold voltage is greater and a second executionmode in which the amount of change in the threshold voltage is smaller;and at a point at which a sign of the offset voltage inverts, or at apoint at which a level of the offset voltage becomes less than atolerance, the control section switches the first execution mode to thesecond execution mode

This configuration makes it possible to quickly cancel offset of thedifferential signal while making no sacrifice of the precision of theoffset cancellation processing.

It is preferable that in the optical receiver according to the presentembodiment, at a point at which a sign of the offset voltage inverts, orat a point at which a level of the offset voltage becomes less than atolerance, the control section reduces a frequency at which the offsetcancellation processing is executed.

This configuration makes it possible to reduce the load of the abovecontrol section while making no sacrifice of the speed of the offsetcancellation processing.

It is preferable that the optical receiver according to the presentembodiment include two or more sets of the transimpedance amplifier, thedifferential amplifier, and the no-signal detection circuit, the two ormore sets corresponding to two or more channels, respectively; and at apoint at which there is no longer a channel in which a sign of theoffset voltage has not yet inverted, or at a point at which there is nolonger a channel in which a level of the offset voltage has notdecreased to less than a tolerance, the control section reduces afrequency at which the offset cancellation processing is executed.

This configuration makes it possible to reduce the load of the abovecontrol section while making no sacrifice of the speed of the offsetcancellation processing.

It is preferable that in the optical receiver according to the presentembodiment, the control section writes a value of the threshold voltageto a nonvolatile memory either at the time when operation of the controlsection ends or periodically during the operation of the controlsection; and at the time when the operation of the control sectioncommences next, the control section reads out, from the nonvolatilememory, the value of the threshold voltage, and uses, as an initialvalue for the offset cancellation processing, the value of the thresholdvoltage that has been read out.

This configuration makes it possible to quickly cancel offset of theabove differential signal even in a case where power of the aboveoptical signal changes over time concurrently with deterioration, overtime, of a light emitting element provided in an optical transmitter, orconcurrently with some other factor.

It is preferable that in the optical receiver according to the presentembodiment, the no-signal detection circuit includes a clear-on-readregister in which a value of an LOS signal is stored, the LOS signalindicating the no-signal section; and the control section reads out thevalue of the LOS signal and identifies the non-signal section byreferring to the value of the LOS signal.

This configuration makes it possible to prevent the control section fromexecuting the threshold voltage change processing in a signal sectionwhose duration is shorter than a cycle of readout, by the controlsection, of the value of the LOS signal. For example, in a case wherethe present embodiment is applied in serial communications inconformance with SAS, it is possible prevent the control section fromexecuting the threshold voltage change processing in a section in whicha COMINIT signal, a COMSAS signal, and an SNT signal is received.

Note that an active optical cable including the above optical receiveris also within the scope of the present embodiment. In such an activeoptical cable, it is possible to estimate, in advance, a range offluctuation of received power in one connector (i.e., in a connectorfunctioning as the optical receiver), such an estimation being based onthe temperature characteristics of a light emitting element included inthe other connector (i.e., in another connector functioning as anoptical transmitter). Because of this fact, in the active optical cable,it easy to realize the offset cancellation processing that uses apredetermined program. Thus, the active optical cable is suited forapplication of the present embodiment thereto.

A control method for the optical receiver according to the presentembodiment is a control method for an optical receiver, the opticalreceiver including: a photodetector element that converts an opticalsignal into an electric current signal; a transimpedance amplifier thatconverts the electric current signal into a voltage signal; and adifferential amplifier that converts the voltage signal into adifferential signal, by differential amplification of a differencebetween the voltage signal and a threshold voltage, the control methodcomprising the step of repeatedly executing offset cancellationprocessing, the offset cancellation processing including thresholdvoltage change processing in which the threshold voltage is changed suchthat an offset voltage of the differential signal is reduced, thethreshold voltage change processing being skipped in a no-signal sectionof the optical signal.

In other words, a control method for the optical receiver according tothe present embodiment is a control method for an optical receiver, theoptical receiver including: a photodetector element that converts anoptical signal into an electric current signal; a transimpedanceamplifier that converts the electric current signal into a voltagesignal; and a differential amplifier that converts the voltage signalinto a differential signal, by differential amplification of adifference between the voltage signal and a threshold voltage, thecontrol method including the steps of: (i) detecting a no-signal sectionof the optical signal; and (ii) performing control by repeatedlyexecuting offset cancellation processing, the offset cancellationprocessing, when being performed outside the no-signal section,including threshold voltage change processing in which the thresholdvoltage is changed such that an offset voltage of the differentialsignal is reduced, and the offset cancellation processing, when beingperformed in the no-signal section, not including the threshold voltagechange processing.

The above configuration makes it possible to cancel offset of thedifferential signal while causing no distortion of a waveform of avoltage signal immediately after the start of a signal section thatfollows a no-signal section.

[Additional Remark]

The present invention is not limited to the embodiments describedherein, but can be altered by a skilled person in the art within thescope of the claims. That is, an embodiment derived from a propercombination of technical means appropriately modified within the scopeof the claims is also encompassed in the technical scope of the presentinvention.

INDUSTRIAL APPLICABILITY

An optical receiver according to the present invention can be usedsuitably in serial communications in conformance with standardsspecifying transmission and reception of an OOB signal, a signalincluding an EI section, or the like, for example, SAS 2.0 or PCIe 3.0.

REFERENCE SIGNS LIST

-   -   1 Optical receiver    -   11 Photodetector element    -   12 Receiving circuit    -   12 a Transimpedance amplifier    -   12 b Variable voltage source    -   12 c Dummy transimpedance amplifier    -   12 d Differential amplifier    -   12 e Differential amplifier    -   12 f Differential amplifier    -   12 g Differential amplifier    -   12 h Low-pass filter    -   12 i Switch    -   12 j Error amplifier    -   12 k LOS detection circuit (No-signal detection    -   circuit)    -   12 m I2C interface    -   13 MCU (Control section)    -   13 a Voltage readout section    -   13 b LOS readout section    -   13 c Threshold voltage changing section    -   13 d Channel switching section    -   13 e I2C interface    -   14 Nonvolatile memory    -   15 Reference voltage source    -   100 Active optical cable

What is claimed is:
 1. An optical receiver comprising: a photodetectorelement that converts an optical signal into an electric current signal;a transimpedance amplifier that converts the electric current signalinto a voltage signal; a differential amplifier that converts thevoltage signal into a differential signal, by performing differentialamplification of a difference between the voltage signal and a thresholdvoltage; and a control section that repeatedly executes offsetcancellation processing, the offset cancellation, processing includingthreshold voltage change processing in which, the threshold voltage ischanged such that an offset voltage of the differential signal isreduced, at a point at which a sign of the offset voltage inverts, or ata point at which a level of the offset voltage becomes less than atolerance, the control section reducing a frequency of occurrence ofexecution of the offset cancellation processing.
 2. The optical receiveraccording to claim 1, wherein: the optical receiver includes two or moresets of the transimpedance amplifier and the differential amplifier, thetwo or more sets corresponding to two or more channels, respectively;and at a point at which there is no longer a channel in which a sign ofthe offset voltage has not yet inverted, or at a point at which there isno longer a channel in which a level of the offset voltage has notdecreased to less than a tolerance, the control section reduces afrequency of occurrence of execution of the offset cancellationprocessing.
 3. The optical receiver according to claim 1, wherein; theoffset cancellation processing has two execution modes which aredifferent from each other in an amount of change in the thresholdvoltage during the threshold voltage change processing, the twoexecution modes including a first execution mode in which the amount ofchange in the threshold voltage is greater and a second execution modein which the amount of change in the threshold voltage is smaller; andat, a point at which a sign of the offset voltage inverts, or at a pointat which a level of the offset voltage becomes less than a tolerance,the control section switches the first execution mode to the secondexecution mode.
 4. The optical receiver according to claim 1, whereinthe control section writes a value of the threshold voltage to anonvolatile memory either at the time when operation of the controlsection ends or periodically during the operation of the controlsection; and at the time when the operation of the control sectioncommences next, the control section reads out, from the nonvolatilememory, the value of the threshold voltage, and uses, as an initialvalue for the offset cancellation processing, the value of the thresholdvoltage that has been read out.
 5. The optical receiver according toclaim 1, further comprising a variable voltage source that generates thethreshold voltage, the control section changing the threshold voltage bycontrolling the variable voltage source.
 6. An active optical cablecomprising an optical receiver according to claim
 1. 7. An opticalreceiver comprising: a photodetector element that converts an opticalsignal into an electric current signal; a transimpedance amplifier thatconverts the electric current signal into a voltage signal; adifferential amplifier that converts the voltage signal into adifferential signal, by performing differential amplification of adifference between the voltage signal and a threshold voltage; and acontrol section that repeatedly executes offset cancellation processing,the offset cancellation processing including threshold voltage changeprocessing in which the threshold voltage is changed such that an offsetvoltage of the differential signal is reduced, the offset cancellationprocessing having two execution modes which are different from eachother in an amount of change in the threshold voltage during thethreshold voltage change processing, the two execution modes including afirst execution mode in which the amount of change in the thresholdvoltage is greater and a second execution mode in which the amount ofchange in the threshold voltage is smaller, and at a point at which asign of the offset voltage inverts, or at a point at which a level ofthe offset voltage becomes less than a tolerance, the control sectionswitching the first execution mode to the second execution mode.
 8. Theoptical receiver according to claim 7, wherein: the control sectionwrites a value of the threshold voltage to a nonvolatile memory eitherat the time when operation of the control section ends or periodicallyduring the operation of the control section; and at the time when theoperation of the control section commences next, the control sectionreads out, from the nonvolatile memory, the value of the thresholdvoltage, and uses, as an initial value for the offset cancellationprocessing, the value of the threshold voltage that has been read out.9. The optical receiver according to claim 7, further comprising avariable voltage source that generates the threshold voltage, thecontrol section changing the threshold voltage by controlling thevariable voltage source.
 10. An active optical cable comprising anoptical receiver according to claim
 7. 11. A control method for anoptical receiver, the optical receiver including: a photodetectorelement that converts an optical signal into an electric current signal;a transimpedance amplifier that converts the electric current signalinto a voltage signal; a differential amplifier that converts thevoltage signal into a differential signal, by performing differentialamplification of a difference between the voltage signal and a thresholdvoltage; and a control section that repeatedly executes offsetcancellation processing, the offset cancellation processing includingthreshold voltage change processing in which the threshold voltage ischanged such that an offset voltage of the differential signal isreduced, the control method comprising: at a point at which a sign ofthe offset voltage inverts, or at a point at which a level of the offsetvoltage becomes less than a tolerance, reducing a frequency ofoccurrence of execution of the offset cancellation processing.
 12. Acontrol method for an optical receiver, the optical receiver including:a photodetector element that converts an optical signal into an electriccurrent signal, a transimpedance amplifier that converts the electriccurrent signal into a voltage signal; a differential amplifier thatconverts the voltage signal into a differential signal, by performingdifferential amplification of a difference between the voltage signaland a threshold voltage; and a control section that repeatedly executesoffset cancellation processing, the offset cancellation processingincluding threshold voltage change processing in which the thresholdvoltage is changed such that an offset voltage of the differentialsignal is reduced, the offset cancellation processing having twoexecution modes which are different from each other in an amount ofchange in the threshold voltage during the threshold voltage changeprocessing, the two execution modes including a first execution mode inwhich the amount of change in the threshold voltage is greater and asecond execution mode in which the amount, of change in the thresholdvoltage is smaller, the control method comprising: at a point at which asign, of the offset voltage inverts, or at a point at which a level ofthe offset voltage becomes less than a tolerance, switching the firstexecution mode to the second execution mode.